Protective device



D. T. MAY.

PROTECTIVE DEVICE. APPLICATION FILED .IAN.1B. l9l8- Patented Oct. 28,, 1919.

2 SHEETS-SHEET I- Brm kdavm -volts D C D. T. MAY.

PROTECTIVE DEVICE.

APPLICATION HLED JAN. 1B. 1918.

Patented Oct. 28, 1919.

2 SHEETS SHEET 2.

Davin r. MAY, on NEW YORK, N. Y.,

INCORPORATED, on NEW YORK,

ASSIGNOR T WESTERN ELECTRIC COMPANY,

N, Y., A CORPORATION OF NEW YORK.

PROTECTIVE DEVICE.

I ToaZZ whom it may concern:

I voltage of a gap Be it known that I, DAVID T. MAY, a citizen of the United States, residing at New York, in the county of New York, State of.

New York, have invented certain new and useful Improvements 1n Protectlve Dev ces,

of which the following is a full, clear, conelse, and exact description.

his invention relates to devices for protecting electrical circuits from abnormal voltages, and more arresters which comprise a discharge gap in a vacuum.

One requisite of an arrester for use in connection with telephone and like circuits is that its discharge or breakdown voltage shall be somewhat less than 550 volts is to say, whensubjected to such a voltage, the device will permit the discharge of electricity across its gap, thus providing a shunt past the apparatus to be protected As is well known, the discharge or breakdown decreases as the air pressure a certain critical pressure which a further reduction is reduced untll is reached, after in pressure causes the rapid rise in the mincreased. This critical nnum voltage that can overcome the resistance of the gap. It is therefore desirable to maintain at the gap a degree of vacuum as near as practicable to the critical pressure for the length of the gap selected.

A further requirement to be met is that the high-voltage protective value of the arrester should be high; that is to say, highvoltage, high-frequency currents will pass through the arrester rather than through the apparatus to be protected. This highvoltage protective value is customarily denoted in terms of the equivalent spark gap or the E. S. G. of the arrester, that is, the length of the gap in air between two elec trodes in parallel with the arrester ga which will be refused by high-voltage, highfrequency discharges in favor of the arrester gap. Evidently, the smaller the equivalent spark gap of an arrester, the greater will be itsprotective value. As in the case of the discharge voltage, there is, for a given spacing of selected electrodes, a critical pressure for the equivalent spark gap at which it has a minimum value, increasing therefrom whether the pressure be increased or depressure does not, however, as a rule, coincide with the breakdown voltage critical pressure," the former Specification of Letters Patent.

particularly to lightning- D. (1, that Patented Oct. 28, 1919.

Application filed January 18, 1918. Serial No. 212,471.

therefore, the compromise intermediate pressure at which the arrester is used needs to differ but little from either critical pressure.

have discovered that this result may be obtained by maintaining a magnetic field longitudinally across the gap between the electrodes.

The means whereby this is accomplished, and further advantages resulting therefrom, will be apparent from the following detailed description taken in connection with the accompanying drawings in which Figure 1 illustrates one form of a vacuum arrester embodying this invention, the front of the container being broken away; Fig. 2 is a view partly in cross-section of a modified arrester; and Fig. 3 is a chart graphically showing the phenomena involved.

In the arrester shown in Fig. 1 there is provided an evacuated container 2, through opposite ends of which are sealed leading-in conductors 4. A plate electrode 6 of brass or other suitable metal, is connected to the inner end of each conductor, the two elec trodes facing each other and being separated. by a sheet of insulating material 8, such as mica which is shorter than the electrodes, so as to leave at each end a discharge gap 10.

The electrodes 6 are held in fixed spaced par' allel relation by means of the insulating p .sheet 8 and a pair of clamping plates 12 connected by bolts 14 and separated from the electrodes by insulation 16. On the rear nets may be employed, that above described being selected because the brass plates 6 form better electrodes than would the faces of the steel magnets; moreover, such electrodes will take almost all of a discharge and thus prevent the discharge from demagnetizing the magnets 18. V Centering washers 24 mounted at opposite ends of the assembled electrodes and magnets prevent lateral displacement thereof. p

In the modification shown in Fig. 2 the construction is the same as that above described, except that the electrodes 7 have their ends 9 out-turned to provide comparatively wide discharge gaps 11, while the insulating separator 15 is divided into two spaced parts whereby there is afforded a central discharge gap 13 substantially shorter than the gaps 11. The normal pressure in the container will be so close to the critical pressure of the gaps 11, that the discharge will take place across one or both of these gaps; while, if because of leakage or otherwise, the gas pressure rises considerably above the critical pressure of the gaps 11, the discharge will not be prevented but will take place across'the gap 13, since the critical pressure rises as gap length decreases. In this form, by the use of the .magnets, the normal pressure of the device may, as explained above, be brought much closer to the critical pressure of the gaps 11, and the certainty of operation of the arrester under high voltages will be increased.

The chart of Fig. 3 shows graphically the equivalent spark gap and breakdown values corresponding to different pressures for an arrester having a discharge gap of one thirty-second of an inch, which has been found to be the most satisfactory length in arresters of this general type. As shown thereon, the discharge or breakdown voltage critical pressure is about four millimeters of mercury, while the equivalent spark gap critical pressure is approximately twenty-five millimeters of mercury when the gap is unmagnetized. Assuming that the actual compromise pressure selected is that indicated by the ordinate A B, namely, fifteen millimeters of mercury, which corresponds to a satisfactory E. S. G. value, namely, about twenty-four thousandths of an inch, it will be seen that the breakdown voltage is nearly 500 volts.

When a magnetic field is produced longitudinally across the gap, the E. lues for the given length ed, as indicate he equivalen .ng lowered to a s of mercury. As a 0 i1 pressure in the gap contai e critical equivalent spark gap press the breakdown voltage being substantiall inchanged;

or it may be reduced to a pressure corresponding to the equivalent spark gap voltage used with the unmagnetized gap, thus reducing the breakdown voltage; or an intermediate pressure may be chosen. The second course will generally be followed, as it is usually most desirable to reduce the breakdown voltage, the protection against high voltages by present arresters having unmagnetized gaps being considered suflicient for most installations. Therefore, in the example given, a pressure of about six millimeters of mercury, as indicated by the broken line ordinate CD, will generally be used, at which pressure both the discharge voltage and the equivalent spark gap have desirable values.

What is claimed is:

1. In a protective device for electric circuits, a partially evacuated container,

mally corresponding to the spacing of said electrodes at the gas pressure in said container.

3. In a protective device for electric circuits, a partially evacuated container, a pair of spaced electrodes in said container, and means within said container for producing a magnetic field between said electrodes.

4. In a protective device for electric circuits, a partiallyevacuated container, a pair of spaced electrodes insaid container, and means within said container for producing between said electrodes a magnetic field extending in the direction of discharge between said electrodes.

5. In a protective device for electric circuits, a partially evacuated container. a pair of plate electrodes in said container facing each other in spaced relation, and a magnet mounted on the rear side of each electrode with its pole faces adjacent the ends thereof and opposed to unlike pole faces on the magnet on the other electrode.

6. In a protective device forelectric circuits which comprises two alternate discharge gaps, means producing a magnetic field at one of said gaps.

7.-In a protective device for electric circuits comprising two alternative discharge gapsof different lengths, means producing a magnetic field at the longer gap.

8. In a protective device for electric circuits, a partially evacuated container, apair of plate electrodes in said container facing each other in spaced relation, the end portions of said electrodes being spaced farther apart than their central portions, whereby said end portions and central portions form discharge gaps of different lengths, and a magnet mounted on the rear side of each electrode with its pole faces adjacent the ends thereof and faces on the magnet on the other electrode.

In Witness whereof, I hereunto subscribe my name this 12th day of January A. D., 1918.

DAVID T. MAY.

opposed to unlike pole 10 

